When Obama gives the signal that he is ready to make the announcement, it won't be done with a noisy news conference. Instead, a plan is in place to e-mail his choice to supporters and the media simultaneously. A campaign tour through swing states with the nominee will soon follow.

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The best-kept secret has spawned almost daily rumors and theories about Obama's choice and his thinking process, often amusing Obama when he hears them.

Although a dark-horse candidate is always possible, aides suggest that Obama has pared his short list down to Biden, Bayh and Kaine. Each comes with advantages and disadvantages for the Democratic nominee.

Biden, chairman of the Senate Foreign Relations Committee, comes with pages of experience with foreign affairs, and he just returned from personally assessing the showdown between Russia and Georgia. He would give an Obama ticket heft on foreign policy, something that Obama's resume lacks. He is also a Catholic and has appeal to white, working-class voters.

But having been first elected to the Senate in 1972, Biden also could be tagged as a Washington insider, and is hardly a fresh face , which would clash with the Obama theme of championing change. And while his garrulous nature could be an asset come October, he can be gaffe-prone, as when he infamously referred to Obama last year as "articulate" and "clean."

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Bayh, a popular former governor of Indiana, a state Obama desperately wants to win, also bolsters an Obama ticket on national security issues. He is relatively young and telegenic and, as a supporter of Sen. Hillary Clinton during the primaries, could possibly help bridge the gap with Clinton supporters.

Democratic strategists are not sure how well the soft-spoken and often bland Bayh would fare in the traditional veep campaign role of being the ticket's attack dog. And his hawkishness on Iraq has earned him enmity among liberal activists.

Kaine and Obama have great chemistry, aides to both men say, and Kaine was one of the first elected officials outside the state of Illinois to endorse Obama's presidential run. But the former mayor of Richmond, like Obama, is a newcomer to national politics and could leave the Democratic ticket open to charges that they are too inexperienced.

Still not counted out altogether is a dark horse candidate such as Clinton, the New York senator who battled Obama for the nomination even after it appeared she had little or no chance to win. Her appeal among women and blue-collar voters, and their lingering disappointment at her loss, could still make her a compelling running mate.

ABC News' chief Washington correspondent George Stephanopoulous, however, pegged her chances at 50-1.